31 December 2013

Anyone who has read 3 Nephi 8, especially if they are aware of some of the details of Mount St Helens' 1980 eruption, have pondered expressions such as "...there were exceedingly sharp lightnings...", "...the city of Moroni did sink into the depths of the sea...", "...the whole face of the land was changed...", "...there was darkness upon the face of the land...", and "...the inhabitants thereof who had not fallen could feel the vapor of darkness..." that engulfed the Nephites nearly 2,100 years ago.

Central America, of course, is an integral part of the Pacific Ring of Fire, so-called because of the string of volcanoes that all lie just inland from the Pacific Ocean margins. The Ring includes hundreds of volcanoes, some of them HUGE, like Cerro Hudson in southern Chile, Masaya in Nicaragua, Shasta in California, Mount Rainier in Washington, Mount Edgecumbe near Sitka, Alaska, and Katmai and Veniaminof, the monster volcanoes of the Aleutians. Farther east, on the other side of the Pacific Ocean, the Ring of Fire includes Bezymiani, Sheveluch, and Mutnovski-Gorely in Kamchatka, and Alaid and others in the Kuriles. The Ring includes Usu, Fuji, and Sakura-Jima, the best-known volcanoes in Japan. We can't leave out Mount Pinatubo in the Philippines, whose 1992 eruption lowered the world wide temperature by two degrees centigrade, and we must include the long arc of volcanoes in Indonesia, including the monster Toba. The phenomenal eruption of the Toba supervolcano around 72,000 years ago may have reduced the proto-human population on Earth to less than 10,000 individuals according to genetic studies.

All these volcanoes (except for Indonesia) lie just inland of the Pacific Ocean margins because they lie just above their sources: the down-going Pacific Ocean seafloor that is being over-ridden by continental margins all around it. Linking the over-riding continental plates with their subducted oceanic plate are huge subduction faults. These are the sources of the largest earthquakes in Earth's recorded history, including the magnitude 9.5 Valdivia earthquake of 1960 in Chile (whose tsunami destroyed downtown Hilo, Hawai'i, about 8 hours later). Other subduction earthquakes include the magnitude 8.7 to 9.2 Cascadia event of 1700, which sank an entire forest in Puget Sound, and whose "Orphan Tsunami" destroyed villages on the Japanese east coast. The magnitude 9.0 Tohoku Earthquake of 2010 triggered the meltdown of the Fukushima-Di-Ichi nuclear plant and devastated the northeastern Japanese coast. The huge magnitude 8.6 Aceh subduction earthquake of 2004 created a tsunami that killed at least 250,000 people along the Indian Ocean margins.

During the Spanish era, a number of regional Central American capitals such as Santiago de Guatemala and Nicaragua, Honduras, were repeatedly destroyed and rebuilt. To say that earthquakes and related volcanic tephra-falls changed the face of the land in Central America would be an understatement.

Since the 1963 eruption that created the island of Surtsey, Iceland, and the 1980 eruption of Mount St Helens, volcanologists have known that lightning storms are closely associated with Plinian eruptions. This is because of the vast electric charge dragged aloft along with the prodigious amounts of volcanic ash that are blasted up to the stratosphere.

But what caused the "vapor of darkness" described by Nephi? This was almost certainly a smothering blanket of volcanic ash. Mount St Helens, 1980, was a relatively small (VEI 5) eruption. This event lofted about 3 cubic kilometers of material, and left nearly a meter-thick blanket of ash in Yakima, Washington, 244 kilometers to the east, within a few hours of its eruption.

To get a handle on a smoking gun for 3Ne:8, we must examine the largest volcanic eruptions in Central America. One way to do this is to accumulate information on tephra falls that reached out great distances - the larger the reach, the greater the eruption. Two events stand out:

Masaya volcano, Nicaragua, about 2,100 years ago, left tephra as far as 170 km distant.

Chiletepe volcano, Nicaragua, about 1,900 years ago, left tephra as far as 570 km distant.

Note that these dates are somewhat approximate (they come from Kutterolf et al, 2008, Geochemistry, Geophysics, Geosystems). The Masaya eruption lofted approximately 8 cubic kilometers of ash and tephra, nearly three times more than Mount St Helens. Interestingly, ancient human footprints have been found at Acahualinca - these are ~2,100-year-old fossils discovered along the shores of Lake Managua, Nicaragua, frozen in the volcanic ash blown out from Masaya. Both these volcanoes lie eastward of the subduction zone where the Cocos Plate is being over-ridden by the Caribbean Plate at a rate of nearly 7 cm/year. This rate is nearly three times faster than the Cascadia subduction rate, which means that there are proportionally more frequent earthquakes and volcanic eruptions in Nicaragua than in Washington and Oregon.

22 December 2013

According to my calculations, the 6th grade means students are around
11-12 years old. If so, then the Rising Generation is full of people a
lot smarter than I was at that age. The question below from Ask-a-Geologist is just one of
many like it:

Q: Dear Geologist,

Our name is Arianah and Cray and we are sixth grade students
at Preston Middle School in fort Collins, Colorado. We are currently learning
about how the Earth’s surface changes over time. We are curious about
earthquakes. We have a couple questions for you. Is there a common time when
earthquakes happen during the day? Also, why did you become a geologist?

Yours sincerely, Arianah and Cray :D

A:

1. Earthquakes are essentially random. We understand why they happen, we understand where they happen, but we do NOT understand WHEN
they will happen. There are always
aftershocks following a main event, of course, but the main event cannot
be predicted. Extensive research has shown that there is no correlation
between earthquakes and certain times of the day or external *
events - for instance there is no
correlation with either the location of the Sun, or of the Moon, or with
tides (alignments of celestial bodies, which cause neap tides or spring
tides, is called syzygy).
Some of the brightest minds on this planet have been searching for more
than a
half century for some evidence that main event earthquakes can be
predicted, but without
success. They can be forecast #, but not predicted.

2. I was a solid-state physicist and realized that if I didn’t
do something drastic, I would be stuck inside a laboratory all my life
with
radioactive sources and high-pressure cells. This was brought very much
to my attention one day when I had a high-pressure cell blow out and
spew Cobalt-60 all over the inside of our lab, and had to call in a
special
Spill Team. Also, by this time physics as a profession was drifting into
a dead end with string
theory, and I saw relatively little value to humanity to spending
billions of dollars to see if
another exotic particle existed. I checked out breakoffs of physics,
including
astrophysics, hydro-geophysics, weather physics, and geophysics, and
found the
last one to be very exciting. It also got me out into exotic places,
like the
Venezuelan jungle, the southeastern Alaska panhandle, the Empty Quarter
of
Saudi Arabia, etc. Geoscience gives me amazing opportunities to visit
these places and many more. But even more interesting to me is to be a
detective – to be
the first to discover something beneath the ground or the seafloor. I
was the first to say where the groundwater was beneath the San Pedro
Basin in Arizona and Sonora, Mexico, and the first to map where titanium
sands lay beneath the seafloor off the coast of South Africa. That’s
ever so cool.

* It has been shown that if you inject fluids into certain
formations (e.g., deep sediments northeast of Denver, CO), you can
trigger swarms of micro-earthquakes. Basically this is the ground
shuddering to equilibrate and adjust itself to a slightly new stress
regime. However these sorts of events are so small that they are almost
never felt.They really are not earthquakes as the general public
understands earthquakes.

# A forecast: in other words, there is an X% chance
that there will be a magnitude Y event on the Z fault zone in northern
California within the next 30 years. This is very, very different from
saying that there will be a Magnitude Y event at Z location on X day -
that would be a prediction. We can't do that.